Sim card switched from iPad to iPhone?

Hi guys, I need help on this questions which bothered me so long time.

Look, I had at&t grandfathered unlimited data plan on my iPad mini 4, the LTE network which is never throttle. I tried to put this sim card from iPad into my unlock iPhone 6s Plus, and the size was exactly fit.

Then the problems came. My iPhone has a signal as at&t still, and the network show as 4G. But I could never get onto internet it means the network doesn't work at all.

How can I make it work? Should I change any setting in the phone or maybe somethings else? I don't wanna use any voice servers on my iPhone, just want to use it like an iPad, that's all.

Finally, I got my iPhone's LTE network worked with my iPad at&t unlimited data plan sim card. Also my iPhone won't make any voice call, but that was not what I cared about. To use the service just visit unlockit.co.nz from your iPhone and follow the instructions on screen. For iPad sim card you have to choose at&t broadband option, that's it.

Finally, I got my iPhone's LTE network worked with my iPad at&t unlimited data plan sim card. Also my iPhone won't make any voice call, but that was not what I cared about. To use the service just visit (address removed) from your iPhone and follow the instructions on screen. For iPad sim card you have to choose at&t broadband option, that's it.

Thank you anyway for the help.

I was reading an article about the process of doing that from that site are not secure. Granted the article was a bit old, I could find no other articles that state it's been updated so that man-in-the-middle attacks cannot occur.